A rare earth magnet has been used in various technical fields. For example, a stepping motor includes a rotor, which is provided with a permanent magnet made of a rare earth magnet on an outer peripheral face of a rotation shaft, and a cylindrical stator in which a plurality of pole teeth facing an outer peripheral face of the permanent magnet is disposed in a circumferential direction, and a rotation angular position of the rotor is controlled by an exciting current which is supplied to a coil of the stator.
A permanent magnet made of such a rare earth magnet is obtained, for example, so that a formed body including magnetic powder which contain rare earth metal is formed and then the formed body is magnetized. Further, in the rare earth magnet, a bond magnet has a structure in which magnetic powder is solidified by binder resin, and a sintered magnet has a structure in which magnetic powder is solidified by sintering.
A magnet (Nd—Fe—B) which uses neodymium among rare earth metals is provided with extremely superior property as a magnet but is apt to rust. When magnetic powder of rare earth metal is rusted and dropped off, the rare earth magnet may be collapsed.
Therefore, a technique in which a coating film is formed on a formed body to prevent occurrence of rust and a technique in which rust preventive liquid is impregnated into a formed body to prevent occurrence of rust have been proposed (see Japanese Patent No. 2879645 and Japanese Patent Laid-Open No. Hei 11-238611).
A rare earth magnet which is formed by compression molding is provided with a large number of pores and rare earth metal is exposed on an outer face of the rare earth magnet and on the pores in the inside of the rare earth magnet. Therefore, even when a coating film is formed on an outer face of a formed body, the coating film may not be formed in the inside of the pore and, in this case, the rare earth metal may be rusted in the inside of the pore.
Further, in the rare earth magnet, even in a case that rust preventive material is impregnated, when powder of rare earth metal (magnetic powder) is dropped off from the surface of the formed body, rust may occur on a newly exposed powder of the rare earth metal.